One from eBay that the seller claims it was taken from a Blast is a good start.
It looks like other vendors are selling rebuild kits as well.
If you must have a genuine Buell brand new one your local Harley dealer can order one from VPI as well.
If you wanted to get creative, you could probably make one fit from a Honda/Suzuki/Yamaha/Kawasaki/Ducati/KTM etc.